1. Field of the Invention
The present invention relates to an abnormality determination device and device for an EGR device, for determining abnormality of the EGR device that recirculates part of exhaust gases exhausted from an internal combustion engine to an intake system thereof, and an engine control unit.
2. Description of the Related Art
Conventionally, there has been proposed an abnormality determination device of this kind in Japanese Patent Publication (Kokoku) No. H07-103831. In a control system of an internal combustion engine described in this publication, an operation for recirculating exhaust gases (hereinafter referred to as “EGR operation”) is carried out by an EGR device depending on operating conditions of the engine. Further, the control system performs an air-fuel ratio feedback control by calculating a feedback-controlled variable, e.g. a fuel injection amount, such that the air-fuel ratio of an air-fuel mixture detected by an air-fuel ratio sensor becomes equal to the stoichiometric air-fuel ratio.
Further, in this control system, the determination of abnormality of the EGR device is carried out as follows: While executing the EGR operation, a feedback-controlled variable is stored which is calculated during execution of the air-fuel ratio feedback control. Next, while continuing the air-fuel ratio feedback control, the EGR operation is temporarily interrupted, and a feedback-controlled variable which is calculated in this state is stored.
Then, the difference between the feedback-controlled variable calculated during execution of the EGR operation and that calculated during interruption of the EGR operation is calculated. If the difference is smaller than a predetermined threshold value, it is determined that the EGR device is faulty. This is because assuming that the EGR device is normal, in accordance with an increase in the amount of fresh air caused by interruption of the EGR operation, the feedback control causes the fuel injection amount to be calculated to be increased so as to make the air-fuel ratio equal to the stoichiometric ratio, which means that the feedback-controlled variable calculated at this time should be larger than a value calculated during execution of the EGR operation.
However, the conventional abnormality determination device has to interrupt the EGR operation, for the purpose of the determination, during the air-fuel ratio feedback control. This makes it impossible to obtain the effect of EGR during the interruption of the EGR operation, causing e.g. an increase in the amount of emission of NOx, and therefore there is a fear that exhaust emissions are increased, and drivability is degraded.